Trial frames of this kind are typically used to determine ophthalmic lenses. To do so, the trial frame is adjusted to a subject, that is a person, and easily exchangeable insert lenses or trial lenses are inserted into the trial frame. The insert lenses are arranged in a rotatable manner so as to be able to correct astigmatic defects in their axial position. Lateral defects and height defects, which are corrected by means of prisms, have to be adjustable as well, which is why the trial frame has to be precisely adjusted to a subject or patient. For example, a pupil distance has to be adjustable, wherein a pupil center of each eye is supposed to be adjustable in a separate manner. A height of the trial frame is adjusted via the nose rest, which is also arranged on the bridge in a pivotable manner. By pivoting the nose rest and adjusting the length of the temples, a distance of the trial frame in relation to the eyes can be adjusted. For adjustment to different ear shapes, the temples are pivotable in height and laterally. A trial frame of this kind is known from EP 0 567 817 B1, for instance.
Since the examining person will always adjust the trial frame immediately on the head of the subject, the operating elements of the trial frame have to be easy to operate and they need to be arranged in a manner that allows ergonomic handling. At the same time, the subject should not be unnecessarily bothered by the operation of the trial frame during adjustment of the trial frame. For instance, catching of the subject's hair in the trial frame and hand movements within the closer field of vision are undesired.
Because of the various adjustment options, the known trial frames are composed of a plurality of individual parts, which makes manufacture of the trial frame complex. For instance, height adjustability of the nose rest typically requires an adjustment gear, said adjustment gear being composed of a plurality of gear elements or gear wheels for converting a rotational adjusting motion into a translational motion of the nose rest at a desired gear ratio.
With the known trial frames, the lens holder devices are displaced by means of threaded spindles arranged within the bridge in order to adjust the pupil distance. For this purpose, spindle nuts are formed or arranged on the lens holder devices. For instance, it is known for a lens holder device to be produced in a plastic injection-molding process and for the threaded nut to be molded to the lens holder device. To do so, a threaded spindle is injection-coated in a mold, the threaded spindle having to be unscrewed from the threaded nut in order to demold the lens holder device. Because of cooling or mold shrinkage of the plastic material, however, the threaded nut may become deformed in such a manner that a thread for the threaded spindle will have to be rethreaded.
Also, in the case of some trial frames, the bridge is made of a metal tube into which a groove is milled. Inner flanks of this groove serve as a longitudinal guide for the lens holder devices, allowing the lens holder devices to be transversally displaced. The guide means thus formed has to exhibit a comparatively precise fit in order to preclude sagittal tilting of the lens holder as far as possible. Hence, after milling of the groove, the bridge has to be routinely refinished in order to form the desired loose fit. These necessary manufacturing steps, too, are relatively time-consuming.
Trial frames are also employed together with polarization filter devices, which can be used for eyesight tests in combination with a polarized display of optotypes. Depending on the type of polarization of the optotypes, circular or linear, corresponding polarization filters are needed on the trial frame. However, these polarization filters require different handling, which means that the trial frame has to be designed according to said handling.